Spherical background image
One approach you could use is to create a special converter scene using cycles.

Place the camera in the center of a cube and give each face of the cube its own material based on one of the face images (honestly: rigging the materials and the UV coordinates will probably be the most time-consuming part of this exercise).  You probably want to use an emission BSDF.

Then change your Camera > Lens from Perspective to Panoramic.  Change the new Type: to Equirectangular (unless you want one of the others).  Then render one frame and you should have an equirecangular version of the scene.

Save that image and you should be able to use it as the environment in your main project.

There is also https://github.com/Samsung/360tools, but I have not investigated the exact command-line for converting from a cubemap to equirectangular, and it might require you to convert to YUV and back.